Olefins, such as ethylene and propylene, have become major feedstocks in the organic chemical and petrochemical industries. While the requirements for ethylene, as a feedstock, is about double that for propylene, fluctuations in the demand for these two materials make it desirable to produce one rather than the other, as required by market demands. Consequently, it would be highly desirable to be able to maximize the production of ethylene, as opposed to propylene, or vice versa, utilizing the same system in the same mode of operation, particularly utilizing a catalyst.
Numerous suggestions have been made for the production of ethylene and propylene from various feedstocks and by a wide variety of processes.
At the present time ethylene and propylene are produced almost exclusively by dehydrogenation or thermal cracking of ethane and propane, naphtha and, in some instances, gas oils. About 75 percent of the ethylene and propylene currently produced in the United States is produced by steam cracking of ethane and higher normally gaseous hydrocarbons derived from natural gas, since natural gas contains from about 5 volume percent to about 60 volume percent of hydrocarbons other than methane. However, in most instances, the content of ethane and higher normally gaseous hydrocarbons in natural gas is less than about 25 percent and usually less than about 15 percent. Consequently, these limited quantities of feedstocks, which are available for the production of ethylene and propylene, must be utilized efficiently. Unfortunately, these processes result in low conversions to olefins and selectively to ethylene, as opposed to propylene, is usually poor. In addition, relatively severe conditions, particularly temperatures in excess of about 1,000.degree. C., are required and such processes are highly energy intensive.
In order to reduce the severity of the conditions and, more importantly, to improve the conversion of normally gaseous feedstocks to ethylene and propylene, numerous processes involving the use of solid contact materials have been proposed. Some of these proposals utilize inert solid contact materials in order to improve contact between the feed hydrocarbons and steam and also to maintain a more even temperature throughout the zone of reaction. In other instances, the solid contact material is catalytic in nature. Such use of solid contact materials, particularly catalysts, have resulted in modest improvements in conversion to ethylene and propylene but the selectivity is improved very little. It is, therefore, highly desirable that improved catalytic processes be developed, particularly processes which increase the selectivity to ethylene and propylene. However, little is understood concerning the manner in which such catalysts function, why certain components are effective while similar components are ineffective and why certain combinations of components are effective and other combinations are not. Obviously, a number of theories have been proposed by workers in the art, but this only adds to the confusion, since it appears that each theory explains why a particular contact material works well but does not explain why similar catalytic materials do not work and why other dissimilar materials are effective. As a result the art of catalytic conversion of hydrocarbons to olefins remains highly unpredictable.
As previously indicated, it would be highly desirable to be able to utilize the same equipment and the same mode of operation to maximize either ethylene or propylene production, as market conditions dictate. However, this is complicated by a number of factors. One difficulty is that both thermal and catalytic processes, in the past, produce substantially larger volumes of propylene than ethylene. Where a catalyst is utilized, it is necessary, in most instances, to change catalysts in order to maximize ethylene and propylene production as desired. It is also possible to shift production from ethylene to propylene or vice versa by the addition of catalyst-poisoning materials to the feed. However, such added materials have a tendency to permanently damage the catalyst, in many cases, to the extent that replacement is necessary. Accordingly, it is generally not possible to repeatedly change from the maximization of ethylene to propylene and vice versa.